I'll ignore the last statement as 30 years of history says otherwise.
However, they don't attack the entire field. They don't have a field stretcher and even if they did Purdy doesn't have a very strong arm. The way to defend them is to get pressure up the middle, maybe using some ILB blitzes or stunts. Force Purdy off his spot and make it impossible for him to step up and set his feet. He is much less effective if he is forced to move sideways. Then just do the normal stuff that we don't usually try with our really good DC. Disrupt the routes in their infancy, give some help behind the initial 7 yards and hope you get to the QB in time. I'm not too worried about Purdy burning us deep. They don't have a Watson, or Love's arm.